Applicability of horse shares to spoils acquired from opening a fortress or city?

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Horse shares are applicable whether the spoils are acquired from opening a fortress, a city, or defeating an army. This is the position of Al-Shafi'i, following the division of the spoils of Khaybar by the Prophet (peace be upon him), which involved fortresses, where the cavalryman received three shares and the infantryman one. Furthermore, horses may be needed if the besieged exit to fight outside.